1. Field of the Invention
This invention relates to optical devices adapted to modulate a first optical signal due to absorption of at least one wavelength of a second optical signal by a material. In particular, changes in the index of refraction as a function of temperature are employed to modulate an optical signal which is totally internally reflected.
2. Description of Prior Art
Typical optical imaging systems can involve the components shown in FIG. 1, that is a detector array 12, readout means 14 coupled to array 12, an analog/digital conversion component 16 and a signal processor 18. System trade offs generally are simple when a few detectors are used. However, when many detectors, for example a focal plane array such as array 12, are employed the problem becomes a complex trade off of speed, resolution and dynamic range. Often optical signal processing provides an answer to the high throughput requirements of the system.
Nevertheless, connecting components may still be a problem. The detectors in array 12 are typically connected via charge coupled device (CCD) readouts (i.e. readout 14) to A/D converter 16. The digital data is then input to signal processor 18. Thus, at least one stage of optical/electrical conversion is involved, and if an optical signal processor is used as processor 18, the process is more complex due to the electrical/optical conversion required to render processor 18 operational.
Further, many optical processors 18 are adapted to process only coherent light. Thus means and methods for direct processing of an optical input signal (i.e. optical/optical processing) is desirable and direct conversion of an incoherent input beam to a coherent output beam is particularly desirable. The technique proposed herein to solve this conversion problem involves total internal reflection.
Total internal reflection has been used as a mechanism for absorption spectroscopy. Prior absorption spectroscopy devices have employed waveguides. Therein, a waveguide having a core of a first index of refraction and a cladding of a second index of refraction, where the second index of refraction is smaller than the first index of refraction, and wherein an absorptive sample is placed in contact with a surface of the core or in close proximity thereto so that the evanescent field of light propagating along the waveguide is partially absorbed by the sample, have formed the basis of a standard absorptive spectroscopy arrangement.
In such prior systems, the total absorption of the sample in close proximity to the waveguide varies because the concentration of absorptive particles or elements in the sample generally varies as the sample is flowed over the waveguide surface. Also, the output from such absorption spectroscopy devices is a sum of the effect on light propogated along the core which comes into close contact with the absorbing sample.
Heretofore, no means or method have been disclosed for providing an output which is sensitive to changes in the index of refraction of a cladding material at a total internally reflecting interface within a preselected area of the interface. This ability to differentiate effects within an area at the interface can provide the basis for adapting a totally internally reflecting device to function as an optical imaging mechanism.